vSphere Replication and Virtual Raw Device Mappings

Some vSphere administrators utilize a storage feature called “raw device mapping” or RDM. There are two types of RDM – virtual RDM and physical RDM. For more information on RDM, please see the vSphere 6.0 Documentation Center. In general, I recommend using VMDK files or Virtual Volumes, but there are certain benefits of RDM.

“Does vSphere Replication support the replication of RDMs?”

The answer is yes, but only virtual RDMs. vSphere Replication does not support physical RDMs. The next question I get is “How is the virtual RDM restored when recovered by vSphere Replication?” The answer is actually quite simple: It is recovered as a VMDK file at the target location. If you would like to see more details, keep reading…

I created a Windows virtual machine with a single VMDK for the C: drive. I have a 200GB LUN mounted by my cluster of hosts, but I did not format the LUN with VMFS. I edited the configuration of the VM by adding an RDM disk (the unformatted 200GB LUN).


We can see the RDM in virtual compatibility mode, i.e., virtual RDM, below. Notice the Disk File has a VMDK extension. However, we can see Hard disk 2 is mapped to a physical LUN at the bottom of the screen shot (you will not see that with a regular VMDK file).


I logged into the Windows virtual machine, configured the newly-added storage, formatted the volume with NTFS and made it the R: drive, and created a folder and a file on the new volume. I logged out of Windows and went back to my vSphere Web Client to configure replication. My target datastore was a Virtual SAN datastore.


After the initial replication sync completed, I recovered the virtual machine using vSphere Replication. Examining the files in the target Virtual SAN datastore, we see the 200GB VMDK file.


I powered on the virtual machine and looked for the folder and file I created on the R: drive. Sure enough, the R: drive was there and the same folder and file were there.

If you wish to maintain the use of a virtual RDM at the target location, it is possible to create a virtual RDM at the target location using the same size LUN, unregister (not delete from disk) the virtual machine to which the virtual RDM is attached from the vCenter Server inventory, and then use that virtual RDM as a seed for replication. However, this process is a bit more cumbersome – especially compared to what we just discussed above. My advice is to keep it simple.